High efficiency modulating and amplifying system



Nova 29 1938., Fe. B. DOME 9 HIGH EFFICIENCY MODULATING AND AMPLIFYING SYSTEM Filed Oct. 29, .1957

To source of carrier frequency To source of audlo frequencg To sourceof audio frequency To source of CGTTIGI" f r'equenc 7 To source 4- of audio frequency Inventor: Robert B. Dome,

His Attorney.

Patented Nov. 29, 1938 UNITED STATES HIGH EFFICIENCY MODULATING AND AM- PLIFYING SYSTEM Robert B. Dome, Bridgeport, Conn, assignor to General Electric Company, a corporation of New York Application October 29, 1937, Serial No. 171,675

6 Claims.

My invention relates to modulating and amplifying systems for radio transmitters and like apparatus, and particularly to methods of and means for increasing the efiiciency and lowering the first cost and operating cost of modulator and amplifier devices employed, for example, in high power radio transmitters.

It is recognized that the power supply required for the high power amplifier equipment of a radio transmitting station represents a considerable proportion of the station operating cost. With the generally increasing power levels employed in radio broadcasting, the matter of increased efiiciency of the power amplifier stages has, therefore, become of increased importance.

Amplifiers of the linear or similar type have been commonly employed in radio transmitters but have been, at times, replaced by other types of amplifiers, particularly in high power installations since the linear amplifier has not in general been capable of operation at more than a relatively low overall efliciency. By reason, however, of the well known advantages inherent in the use of amplifiers of the linear and similar types in radio broadcasting and other transmitters, it is highly desirable that means be provided whereby this type of amplifier may be retained in such applications.

It is an object of my present invention, therefore, to provide methods of and means for increasing the efficiency of operation of amplifiers of the linear and similar types, to such an extent that they may be retained in radio transmitter installations, of relatively high power for example, wherein it is required that the operating costs be reduced to the lowest possible limit.

In radio transmitters employing conventional amplifier systems of the above and similar types, it is well known that for carrier conditions in the transmitter, or conditions in which only the carrier wave is'transmitted, no modulation being present on the carrier, an efficiency of not more than of the order of thirty to thirty-three per cent may be obtained, if the amplifier is at the same time to be capable of providing an output power corresponding to a 100 per cent positively modulated carrier wave.

It is, therefore, a further and particular object of myinvention to provide methods of and means for increasing the efficiency of the amplifying system in radio transmitters to a very high degree under carrier conditions in the transmitter, thereby reducing to a low level the power supply required for the amplifier system, under carrier conditions, without affecting the ability of the system to provide the output power corresponding to- 100 per cent modulation conditions of the carrier wave.

In carrying my invention into effect I provide means whereby, in a radio transmitter or like apparatus for the transmission or a modulated wave of carrier frequency, an amplifying device or tube operates into a variable modulated load, a portion of the load being radiated on an antenna or consumed in any suitable load circuit, and another portion of the load being rectified, the rectifier current being fed back to replenish the power supply means supplying D. C. power to the amplifying device.

In one embodiment of my invention the rectifying means which provides the feedback of power to the amplifying device is controlled by a quarter-wave transmission line or network section connected in the output circuit of the amplifying device. In a modification of my invention a similar rectifying means is shunt arranged in a coupling circuit provided between the output circuit of the amplifying device and the antenna circuit or other load consuming work circuit.

The novel features which are considered to be characteristic of my invention are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawing, in which Fig. 1 is a diagrammatic representation of a radio transmitter in which my invention has been embodied, and Fig. 2 is a diagrammatic representation of .a radio transmitter embodying a modification of my invention.

In Fig. 1 the numeral I indicates a space discharge device or tube which in the present embodiment of my invention is a power amplifier for modulated carrier waves. This tube may be, for example, of the 207 type. Tube l is arranged to be shunt fed through a choke 2 from a source of anode potential 3, and the anode 4 of the output circuit 5 of the tube is coupled through a blocking capacitor 6 to an oscillatory circuit 1 comprising an inductance 8 resonating with a capacitor 9 at the operating frequency. An antenna H3 is coupled to inductance 8 through a coupling coil H and is tuned by an antenna capacitor l2.

Preferably choke 2 has such a value of inductance that the choke inductance resonates, at the operating frequency, with the capacity of tube l to ground, and with the capacity of connecting wires and the stray capacity of capacitor 6 to ground, the reason for this requirement being that it is essential that the combination 8, 9 be truly resonant and not reactive.

To the lower end of the oscillatory circuit 1 is connected the near end l3 of a quarter-wave section of network [4 which may comprise, for example, an inductance l5 and connected thereacross, two capacitors l6 and I! in series. The

circuit 1, quarter-Wave network section l4, and

cathode l8.

The far end 29 of the quarter-wave network section It is connected through a lead 2'! to the anode 22 of a modulatorrectifier space discharge device or tube 23. The cathode 24 of tube 23 is connected through a lead 25 to the positive terminal 26 of anode potential source 3, and the grid 27 of this tube is connected to cathode .25 through, preferably, a resistor 28, the secondary 29 of an audio frequency transformer 33 the primarytl of which is connected to a source (not shown) of modulating frequencies, and a source of grid bias potential 32. A choke 33 is connested in shunt with capacitor E5 to carry the direct current output of modulator-rectifier tube 23. The output circuit of the modulator-rectifier tube 23 thus includes anode 22, cathode 24, lead 25 connecting cathode 24 to positive terminal 25 of anode potential source 3, potential source 3 to the cathode it of tube 8, choke 33 bypassing capacitor it; of the network section IQ, inductance E5 of the network section i l, and lead 2! connecting far end terminal 2% of network section Hi to anode 22. A choke 3% may in certain instances be shunted across capacitor ll of quarter-wave network section it, for a purpose to be explained hereinafter.

The input circuit 35 of tube l comprises grid or control electrode 36, an optional grid leak 3i and bypass condenser 38, a grid oscillatory circuit 3 comprising inductance lil tuned by capacitor ll, a grid bias potential source t2, and cathode it which is at ground potential. To impress modulated carrier waves on grid circuit 39, this circuit is coupled to the anode circuit 43 of an exciter tube 44, to the input circuit 45 of which is coupled a source (not shown) of carrier frequency, and which is adapted to be plate modulated through an audio transformer 46 coupled to the source of modulating frequencies.

The modulating frequency should be so phased on audio transformers 30 and 56 that the grid of tube 23 goes negative when the modulation is positive on the excitation to tube The system above described is set up for operation as follows: Assuming that a carrier wave is being impressed on tube l, the capacitor is is first short-circuited, thus connecting the lower end of plate oscillatory circuit 1 directly to the cathode of tube 1 and eliminating effectively, for the time being, the quarter-wave network section I4 from the output circuit of the tube. The loading on oscillatory circuit '3 is next adjusted, by adjustment of the coupling to the antenna, until the loading is correct to provide an antenna power equal to four times the desired carrier power. Assuming tube l to be of such characteristics that the plate voltage, EB, supplied by source 3 is 14,000 volts, a load line of 2640 ohms will give an output of 16 kw. with a D. 0. plate current of 1.57 amperes, or an efficiency of =73 percent circuit 7 now including the network section.

Tube 23 is then biased temporarily beyond cutoff, and with the anode-cathode circuit of the latter tube thus made non-conducting, the elements of the quarter-wave network section M are'adjusted until the same 16 kw. antenna power is again obtained. This result in antenna power is obtainable for the reason, in general, that if the far end terminating impedance of a quarterwave network section is infinity, the near end impedance is zero. In the present case, since the far end ill] of network section I4 is connected to the anode 22 of tube 23 which is now nonconducting, the terminating impedance of the far end 29 is infinity and the impedance of the near end 13 is therefore zero. Since the near end impedance of the network is zero, conditions in the output circuit of amplifier i are the same as with oscillation circuit 1 connected directly to cathode it by a short-circuit across capacitor it of network it.

The negative bias on tube 23 is next decreased slowly so that this tube becomes active and begins to rectify the radio frequency voltage across capacitor ill, or across the far end 20 of the quarter-Wave network section M. This action of tube 23 causes the far end impedance of network section it to decrease and the near end impedance correspondingly to increase, The impedance thus introduced, in series with the 2640 ohm load line, in the output circuit 5 of amplifier l is resistive and does not detune the anode circuit of this tube provided the inductance of'choke 2 has the value above mentioned. The lowering of the negative bias of tube 23, and consequent increase in the total impedance in circuit 5, is continued until the antenna power is down to four kw., which as above mentioned is the antenna power corresponding to the unmodulated or carrier conditions of the transmitter. Since, with the antenna power down to four kw., or carrier conditions in the present case, the total plate swing of amplifier l is still approximately the same as before in the 100 per cent modulation conditions of the transmitter, because the excitation to tube l is so arranged that under carrier conditions the plate voltage is given its utmost excursion, therefore the rectifier-modulator tube 23 new places across the far end 20 of the quarter-wave network section l d a load equal to the load across the circuit 1, or 2640 ohms.

The amplifier l under these conditions works into an effective load of 5280 ohms. may vary slightly because the plate swing of amplifier l is slightly increased for increased load lines, and instead of 5280 ohms the value may be about 5600 ohms. The total output of the tube l is now eight kilowatts and the plate current is 0.76 ampere. The efficiency so far as the tube is concerned is now 75 percent Of the total eight kilowatts output, four kilowatts go to the antenna. The other four. kilowatts are rectified by tube 23.

' The high efiiciency of the radio transmitter incorporating the modulating and amplifying system shown in Fig. 1, in accordance with my invention, is due to the arrangement whereby the output of the modulator-rectifier device 23 is fed This value back, by reason of the conniection of cathode 24 through lead 25, to anode potential source 3, the

plate circuit supply of amplifier I, so that of the above-mentioned total input of (14,000) (0.76) or 10.65 kw., four kilowatts is supplied from tube 23, the plate supply source 3 for tube I being called on to supply only 10.65 kw.4 kw.=6.65 kw.

=60.1 percent I This may be compared with values of from, 25 per cent to 35 per cent ordinarily obtainable from low level modulation arrangements now commonly used. In other words, assuming 33 per cent efficiency under carrier conditions, a four kw. transmitter would ordinarily consume %=12.1 kilowatts so that the system herein described, in accordance with my invention, will save, under carrier conditions, almost one half of the plate power ordinarily required.

In operation of the system of Fig. l on modulated carrier waves it will be readily seen that under 100 per cent positive modulation conditions in the system, the modulating audio frequency is so phased on transformer 30 of modulator-rectifier tube 23 and on transformer of the exciter tube 44 that the grid 2'! of tube 23 goes negative to the cut-off point when the modulation is positive on the excitation to amplifier I, and that, therefore, at 100 per cent modulation tube 23 does not rectify any of the modulated R. F. across capacitor ll of the network section M. Correspondingly, the impedance of the far end 20 is infinity and that of the near end I3 is zero. Under these conditions the network Id does not provide any impedance in the output circuit 5 of amplifier I, as before explained 1n connection with the setting-up of the system. It

will further be seen that for carrier conditions in the system, or conditions in which no modulation is impressed on the carrier, the grid bias on modulator-rectifier tube 23 is adjusted initially at such a value that rectification of the radio frequency oscillations across capacitor I! of network l4 occurs sufficient to cause one-half of the total output of amplifier l (8 kilowatts in the present case) to be fed back ,to the plate supply source of the amplifier. Under conditions of per cent positive modulation intermediate the 100 per cent condition and carrier condition it will also be readily understood that the operation of modulator-rectifier tube 23 is such as to cause a feed-back to the plate current of amplifier i appropriate to the particular condition of positive modulation.

It will be understood that for 100 per cent negative modulation the excitation to amplifier I follows the negative envelope of the carrier and that the output antenna current varies correspondingly. Further action of modulator-rectifier tube 23 is arrested in the negative swing of modulation by holding the bias on the latter tube fixed throughout the negative peaks of modulation. Various limiting means for the latter purpose may be employed, preferably the purpose may be accomplished by making carrier conditions correspond to zero bias on tube 23 by providing the audio system connected to transformer 30 with such poor regulation that the grid 21 of tube 23 cannot be driven very far positive. A resistor, 28, may be placed in series with the grid 21 if desired. In order to reduce overall distortion to low values, a portion of the antenna output may be rectified, if desired, and the audio fedback in a negative feedback sense to reduce the tendency toward distortion.

For best results in the system illustrated in Fig. l the constants of the quarter-wave transmission line section !4 should be carefully selected. They may be derived as follows:

The positive reactance of inductance: l5 must equal the negative reactance of capacitor l6, and the reactance of capacitors i6 and H must be equal. Further, it is known from transmission line theory for quarter-wave lines that 3 3 2 or Rz- Where Zu=the surge impedance of the line.

R1=the far end impedance (resistance). Rz the near end impedance (resistance).

Under carrier conditions R2 must be 2640 ohms for the system including the 207 type of amplifier tube cited in connection with Fig. 1, or if this load of 2640 ohms be called R ohms, then As to R1, noting that the output W, of the modulator-rectifier tube 23 under carrier conditions is Wzl kw., and that the D. C. voltage deveolped is Eb=14,000, therefore the equivalent D. C. load resistance Rdc is Now the equivalent A. C. resistance, R1 (which in the present case is the far end resistance of the network section M) is known to be approximately one-half of the D. C. load resistance, or

Since as above stated the positive reactance, oil: of the network section M equals the negative reactance,

of the section, therefore Z =wL (7) Substituting (7) in (5) and solving for L,

w L or L= 5, (8)

Assuming that 1000 kc. operation (middle of the broadcast band) is desired in the system shown in Fig. l incorporating the amplifier of the type above described, then 14000 2640 I L i?10 WV1.28 milhhenries (9) The reactance X, of inductance I5- is,

Capacitors I6 and I l have the same reactance or have each a capacity of C= %.=W =IQ.S micromicrofarads (11) If the capacity of modulator-rectifier tube 23 is in excess of the latter Value, inductance 34 of suitable value may be shunted across capacitor IT, to bring the net reactance up to 805() ohms. Likewise choke 33 may be arranged to resonate partially with capacitor I5 to provide -8050 ohms at that point. Further, inductance i5 may have a distributed capacity which must be taken into account in determining the value of i5, to yield a net reactance of +8050 ohms.

Since on peaks of modulation inductance l5 tunes to resonance with capacitor I'l, its loss must be made low as it adds to the 2640 ohm plate oscillatory circuit load; Assuming that a five per cent loss is tolerable in modulation peaks, the

Q of inductance I 5 must be This value of Q is realizable. inductance l5,Rs, is

X 8050 61 132 ohms The current, I, on peaks of modulation, W0, is that due to five per cent of the peak power, or

The peak voltage, E, across the capacitor H is, therefore,

5:45 IX=1/ (2.45)(8050)=28,000 volts (15) The peak forward voltage across the amplifier l is less than 28,000 volts by the value of the D. C. voltage, or becomes 28,00014,000=14,000 volts. This latter value is not excessive for tubes of the 207 class above mentioned since the tube is rated as a plate modulated amplifier at 10,000 volts, D.'C., which means, at 100 per cent modulation, peaks of 40,000 volts approximately. In the system illustrated in Fig. 1 a UV848 type tube may be employed toadvantage as the modulatorrectifier 23 since as a rectifier it has a lower internal resistance than a 207 type tube.

In a system in accordance with my invention as illustrated in Fig. 1 employing glass amplifier tubes, for example, of the 806 type, it is possible to obtain approximately half the class C rating of the tubes as carrier power with a resulting saving in tube cost over previous systems. A single 806 amplifier tube when incorporated in the present system yields a carrier power of approximately 225 watts.

The modification of my invention illustrated in Fig. 2 is similar to the embodiment illustrated in Fig. 1 in that modulated carrier waves are impressed on an antenna circuit 41 comprising an antenna 48, an inductance 49, and a tuning capacitor 50, from an amplifier tube 5| the excitation of which is supplied from an exciter tube 52 on whose input circuit is impressed a carrier wave from a source of carrier frequency (not shown) and which is plate modulated through an audio transformer 53, the primary of which is connected to a source of modulating frequency (not shown).

The system illustrated in Fig. 2 is further similar to that of Fig. 1 in that the amplifier 5| operates into a variable modulated load, a portion of which is radiated on the antenna 48 and The resistance of 2.45 amperes (14) another portion of which is fed back by a modulator-rectifier device or tube 54 to add to the power supply which furnishes D. C. power to the amplifier. In the system shown in Fig. 2, however, a quarter-wave network section is not employed, in connection with the modulator-rectifier tube, to obtain this result. In Fig. 2 the amplifier output oscillatory circuit 55 is coupled to a resonant coupling circuit 56 comprising inductance 5'! and capacitor 58, in shunt through leads 59 and 60 with a second resonant coupling circuit 6| comprising inductance 62 and capacitor 63 and coupled to the antenna inductance 49. The modulator-rectifier tube 54 is connected in shunt with coupling circuits 56 and 6E. The anode 64 of tube 54 is connected through lead 59 to the high potential side of the coupling circuits 55 and 6! and the cathode 65 is connected through a lead 65 to the positive terminal 5'! of a source (not shown) of anode potential for amplifier 5!,

this source being connected at its positive termi nal 51 to the anode 68 of tube 5i through the inductance 69 of plate tank 55, thus providing series feed for the amplifier tube. Grid ill of the modulator-rectifier 54 is supplied from the source of modulating frequency through a circuit including lead 66, a source ll of bias voltage, and the secondary E2 of an audio transformer 13 the primary T4 of which is connected to the source of modulating frequency.

The circuits are so adjusted that with modulator-rectifier tube 54 biased beyond cut-01f (no plate current in this tube) for 100per cent modulation conditions, the antenna current is twice the antenna current under carrier conditions; under carrier conditions the load on the power amplifier 5 I is half as much, or the load resistance is twice as great, as under the condition of 100 per cent peaks of modulation. Under carrier conditions the modulator-rectifier tube 54 takes onehalf the output of the power amplifier 5i and returns the power to the power supply of amplifier 5! in the form of direct current, similarly to the return of power to the amplifier as described in connection with the embodiment of my invention illustrated in Fig. 1. As in the system of Fig. l, in the system of Fig. 2, the modulating frequency is so phased on the audio transformers (53 and 13 in Fig. 2) that the grid of the modulatorrectifier tube goes negative when the modulation is positive on the excitation to the amplifier tube.

In the system illustrated in Fig. 2, assuming a Value of antenna resistance Ra, this resistance is coupled into inductance 62 as a series resistance, (R1) the value of which is Where Xm1=mutual reactance between inductances 49 and 62.

This resistance appears in turn as a shunt resistance, R2, across inductance 62 of the value,

where X3 is the reactance of inductance 62.

Under carrier conditions the load represented by modulator-rectifier tube 54 provides an additional shunt resistance across inductance 62 of, say, R3 ohms. Thus the total shunt resistance, R4, is

The resistance R4 is transformed to a series resistance, R5, in inductance 51 of the value where X2=the reactance of inductance 51.

The resistance R5 is in turn coupled into inductance 39 as a series resistance, R6, of value where Xm2=the mutual reactance between inductances 69 and 51.

The resistance Rs in turn appears across the tank as a resistance, R7, of a Value Since all of the reactances (Xs) are constant for a given circuit set-up, (26) may be written as k R,,R (27) Now R7 is the resistance load on the power amplifier 5i, and since under carrier conditions k2Ra R3, therefore under the latter conditions l( 2 a+ 2 a) i k312i 1 12, (28

Under 100 per cent positive modulation conditions' R3=, therefore, from (27), under the latter conditions It will be noted that under 100 per cent positive modulation conditions the resistance load (29) is one-half that of (28) representing the load resistance under carrier conditions. This relation between the load resistance for the two above conditions is that required for proper modulation in the system in accordance with my present invention. For the negative half cycle of modulation in the system illustrated in Fig. 2 the grid excitation of amplifier 51 is modulated downward to zero, as described in connection with the system illustrated in Fig. 1.

In the system illustrated in Fig. 2 the various reactances are required to be adjusted so that the modulator-rectifier tube 54 is supplied with 1e proper voltage to rectify one-half of the output of the power amplifier 51 under carrier conditions. These reactances may readily be calculated when the antenna resistance Ba, and the proper load on the amplifier 5| are known. It will be noted that capacitors 63 and 58, parallel to each other and shunted across inductances 62 and 57, may be combined into a single capacitor.

It 'will further be noted in connection with the system illustrated in Fig. 2 that the circuit is simple to set up since no quarter-wave network section is required, that series feed to the plate of power amplifier is possible, and that it is not necessary to tune out the capacity of the modulator-rectifier tube.

My invention has been described herein in particular embodiments for purposes of illustration. It is to be understood, however, that the invention is susceptible of various changes and modifications, and that by the appended claims I intend to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is:-

1. In a transmitting system comprising a source of carrier waves, a load circuit, an amplifier adapted to transfer energy between said source and said circuit and having an output circuit, a source of modulating potentials, means to modulate said carrier waves in accordance with said potentials, and a source of direct current for said output circuit, the method of operation which includes transferring a portion of the load in said output circuit to said load circuit, rectifying another portion of said output circuit load in accordance with said modulating potentials, and adding said rectified portion to the current in said output circuit from said direct current source.

2. In a transmitting system comprising a source of carrier waves, a load circuit, an amplifying device adapted to transfer energy from said source to said circuit and having an anodecathode circuit, a source of modulating potentials, means to modulate said carrier waves in accordance with said potentials, and a source of direct current for said anode-cathode circuit, the method of operation which includes transferring under carrier conditions in said system a portion of the output of said amplifying device to said load circuit, rectifying another portion of said output in accordance with said modulating potentials, and adding said rectified portion to said direct current.

3. In combination in a transmitting system, a source of carrier waves, an amplifying device including an input circuit and an anode-cathode circuit, a load circuit, means to impress said carrier waves on said input circuit from said source, a source of modulating potentials, means to modulate said carrier waves in accordance with said potentials, a current source connected to said anode-cathode circuit to supply direct current thereto, a rectifying device, and means including said rectifying device to transfer under Y iii waves in accordance with said potentials, a source of direct current for said output circuit, and means to add to the current supplied by said direct-current source to said anode-cathode circuit in accordance with said modulating potentials, said last-named means including a quarter- Wave transmission line section connected in series in said output circuit and a rectifying device connected between said transmission line section and the positive terminal of said source of direct current. V

5. In combination in a transmitting system, a source of carrier waves, an amplifying device including an output circuit, means to impress a modulated carrier wave on said device from said source, a source of modulating potentials, means to modulate said carrier waves in accordance with said potentials, a source of direct current for said output circuit, a transmission line section connected in series in said output circuit, a rectifying device connected between the far end of said section and the positive terminal of said direct current source to add current in accordance with said modulatingpotentials from the load in said output circuit to the current supplied to said output circuit from said direct current source, and means including said rectifier to cause under conditions of complete modulation of said carrier wave the near end impedance of said section to be substantially zero and under carrier conditions of said Wave the near end impedance of said section to have a value approximately that of the initial load impedance of said output circuit.

6. In combination in a transmitting system, a source of carrier waves, a load circuit, an amplifying device adapted to transfer energy from said source to said circuit and including an output circuit, means to impress said carrier waves on said device from said source, a source of modulating potentials, means to modulate said carrier waves in accordance with said potentials, a source of direct current for said output circuit, and means to transfer under carrier conditions in said system a portion of the load in said output circuit to said load circuit and to add to the direct current supplied to said output circuit from said current source a direct current in accordance with said modulating potentials, said last-named current being derived from another portion of said output circuit load, said last-named means including a coupling circuit means between said output circuit and said load circuit and a rectifying device connected in shunt with said coupling circuit means and having a cathode connected to the positive terminal of said direct current source.

ROBERT B. DOME. 

